a) Field of the Invention
The invention is directed to a method for generating optically perceptible laser-induced cracks in brittle material as is known generically from DE 101 29 876 C1.
b) Description of the Related Art
Methods for thermal laser beam separation (TLS methods) have been widely practiced for severing brittle material, and there are many publications and patents based on the fundamental method described in DE-AS 12 44 346 which further develop and modify this fundamental method especially to improve the quality of the severing edge and to make the process more efficient.
The various generic methods have in common with the method according to DE-AS 12 44 346 that the material is heated along the desired severing line to a temperature below the melting temperature by passing a laser beam over it, and high thermal stresses are generated along the severing line by a coolant jet following the laser beam such that a crack is driven along the severing line (crack trail) in the material proceeding from an initial crack.
Depending on the particulars of the individual method and the parameters of the method and material, the crack can penetrate completely through the material, particularly when its thickness is small, or may only form a depth crack. Final severing along the severing line is then generally carried out by application of mechanical force along the crack trail immediately thereafter or subsequently so that the material breaks.
Since, in contrast to mechanical cutting processes or laser sublimation methods, there is no removal of material and therefore also no change in the surface structure, the visibility of the severing line is very limited in spite of the crack formation.
In order to ensure a uniform application of force on both sides of the crack trail, a device provided for breaking is oriented to the crack trail. Insofar as the breaking process immediately follows the generation of the crack, the breaking device can be directed so as to be oriented to the guiding of the laser and of the coolant nozzle. Provided that there is no misalignment and the crack is actually still advanced continuously, the breaking edge can be expected to be free from break defects.
However, when the breaking is not to be carried out until later at a location other than that where the crack was generated, it is important that the crack is visible or optically detectable in order that a breaking device or other processing devices for process steps preceding the separation, such as a partial coating or outfitting, can be oriented to the crack trail.
DE 101 29 876 C1 discloses a process by which a permanently visible mark trail is realized along a crack trail (referred to in this case as a scribing trail) in glasses in that a coating is at least partially applied to the crack trail after the laser-induced scribing and prior to the breaking process. Visibility is ensured by dyeing the coating to a color that differs from that of the surfaces of the glass elements and/or by an elevation of the surface on the crack trail attributable to the layer thickness.
It is possible to remove the coating, for example, after the breaking process by washing the individual glass elements. The possibilities suggested for applying the coating to the crack trail (application of a marking trail) are either to thoroughly coat the entire crack trail or to apply the coating at determined distances along the length of the crack trail so that, e.g., only marking points need be placed.
According to DE 101 29 876 C1, the coating can be applied by a jet device, a pen, or a spraying device. When a jet device is used, a liquid which hardens is applied to the crack trail. Application of a coating by means of a jet requires at least one jet device with an application nozzle. According to a described embodiment example, this jet device as well as the device for laser-induced scoring are stationary so that the necessary relative movement is carried out by the glass element.
The described method is disadvantageous in that the marking trail can deviate from the crack trail due to misalignment and the marking trail is applied regardless of whether or not a crack trail has been generated at all. The processing steps following the generation of the crack are not oriented to the actual progression of the crack trail, but rather to a marking trail with a reference path identical to the intended path of the crack trail whose presence is assumed. Discrepancies between the actual paths of the marking trail and of the crack trail lead to fluctuations in the quality of the processing steps following the generation of the crack.
For inspecting cracks, particularly in non-magnetizable workpieces such as ceramics, it is known to use dye penetration methods, as they are called. In this case, workpieces are prepared for crack inspection in a manner known per se in that they are sprayed with dyes, particularly fluorescent dyes, which penetrate into the cracks through capillary forces and therefore are not removed when the surfaces are subsequently cleansed of dye. Accordingly, the cracks are detectable by means of the deposited dyes either immediately or after subsequent treatment with a developer and a predetermined developing time under UV light or visible light by direct visual contact or by automatic image processing. The use of the dye penetration method is known for detecting defects within the framework of quality control.